Automatic telephone system



Feb. 18, 1930. c; E. I OwnmA AUTOMATIC TELEPHONE SYSTEI 2 Sheets-Sheet 1 origmal Filed sept. 16, 192s .Illuwllllnlillllll- @firm larz-Lca .E Lnmsx Feb. 18, 1930. c. E. LoMAx AUTOMATIC TELEPHONE SYSTEM 2 Sheets-Sheet 2 Original Filed Sept. 16, 1925 v l Ilaregnca E Lmmax Patented Feb. 18, 1930 UNITEDl STATES Parral orf-ica CLARENCE E. LoMAx, or CAR PARK, ILLINOIS, `assIeNoR, BY MEsNE AssIGNrs, 'ro AUTOMATIC ELECTRIC INC., or CHICAGO, ILLINoIan cooRA'rIoN or' D- 'WARE AUTOMATIC TELEPHONE SYSTEM The present invention relates to multi-office automatic telephone systems, and the broad general 4object is the production of .an economical inter-oiiice trunking arrangement and an economical arrangement of y,the switches in the various offices concerned while maintaining a uniform and universal numboring arrangement.

Specifically, one obJect ofthe Invention, 1s n, the provision of arrangements for providlng tandem trunking facilities in connection with Referring nowv to the drawings, Fig. 1 is' a trunking lay-out showing four oilices whichl are numbered 22, 33, 44, and 55, respectively, and which comprise a system embodying the featuresof the invention; and Fig. 2 shows the local selector L which is also showndiagramatically in oil-ice number 22, Fig. 1.

The selector L, Fig. 2, is a standard selector in every respect except for the provision of the second-level shaft springs and the associated circuit connections whereby the selector wipers are restored to their normal p0- sition each time the selector responds to the digit 2.

In Fig. 1, the substations A---A8 are shown in the offices 22,333, 55, and 44, respectively.

4c The lines of these substations extend to the line switches liS--LS3 and these line switches have access to the local selectors L-Ls, respectively. In addition to the 4local selectors the incoming selectors I-P are shown. The trunk lines terminatingin the incoming se lectors are two way trunk lines, and they are seized .through the sections associated with the repeaters Tft-R5. This drawing shows also the connectors C-C7, two of these connectors being shown in each office, each having access to a different hundred-line group. It will be noted that certain numbersare placed within the squares representing the incoming selectors andthe connectors. These numbers it may be pointed out represent the digits and the digit combinations that must be dialled in order to extend a connection to the respective switches.

The selectors in the oHice number 22 are arranged 'to drop back when they respond to the digit 2, the selectors in ofiice number 33 are arranged to drop back when they respond to the digit 3, etc. The digit on which eacli selector drops back is indicated by the number placed within the small vc ircle drawn above each selector.` These selectors are arranged to drop back from the critical level repeatedly. That is, if the dialling of the critical digit islfollowed by the dialling of of the. critical digit a second time, the selector drops back the second time also, thus absorbing the critical digit as many times as may be necessary (in the present case two times).

Considering now the trunlng arrangef -ment a little more in detail, it will be-noted that there are trunk-lines directly connect-v ing the oliice 33 with each of the other oliices and that there are no trunk. lines directly connecting the other offices with eachother. The result is that an inter-oiiice call originating in oiiice 33, goes directly to the called oii'ice when the first digit is dialled. Simi-- larl an inter-office call originated in either of t e oies other than oice number 33 and intended for otlice number 33 is trunked to the called office when the first-digit is dialled. However, the inter-oce calls originatin in oices 22,44 and 55 and intended vfor o ces other than oice 33, reach their destination by to which 1t is local.-

way of two ltrunk linesl in tandem, one trunk line extending from the calling oiice to ofice number 33 and the-other trunk line extending fromv office number 33 to the called oice. It will thus be seen that two office digits are required in the latter type of inter-oilice call, the first digit being used to extend the connection to oiiice number-33 and the second digit being used to extend the connection from office number 33 to the called oiiice. It will be seen furthermore that on 'inter-office calls in which oiiice number-33 is a calling oihce or the called oiiice only one digit is required to trunk the calls tothe desired oiice. Now in orderto keep the numbers uniform and universal each office designation contains two digits; namely, 22, 33, 44, and 55. That bein the case, arrangements must be provide to absorb or otherwise 'dispose of the second oilice digit on nontandem inter-oice calls. This result is accomplished by arranging the incoming selectors, including the incoming selectors I--I's so that each will restore its wipers to normal when it is operated responsive to the second di 't in the'designation of the oiiice Inline with the foregoing remarks, it may for example, be noted that when oice number 44 is called throu h the local selector L in oiiice number 22, t e first digit.4 causes the selector L to raise its wi ers oppositethe fourth 'level and select an i e outgoing trunk` throughv ay repeater, such as R, to an incom- 'ing selector,fsuch as I', in oilice number 33. The incoming selector I responds to the second oiiice digit 4 and selects an idle outgoing trunk through a repeater, such as R3, to an incoming selector, such as I5 in oiice number 44. The incoming selector I6 then responds to the next digit, either a 6 or a 7 to extend a connection to one of the connectors, such as C or C", which responds to the last two -digits in the number to extend the connection to. the called line.

It may be seen also thaton a call from one of the outlying oiiices to oiiice number 33, say :from oiiice number 22, the dialing of the first oiiice digit 3 results in the local selector raising its wipers 'opposite the third level and selectingan idle outgoing trunk line through a repeater, such as R, to an incoming selector such as I. The selector I, responds to the secondoilice digit 3 by raising its wipers opposite the third level. However, as is indicated by the numeral 3 placed inside the circle just above the selector I', the selector I releases its wipers whenever it is operated in accordancewith the digit 3. That being the case, when the second oiiice digit 3is received it is wiped out by the incoming selector I when the wipers of this selector are automatically restored to their normal osition. The dialling of the next digit, eit er a `6 or a 7, results in the extension of a connection to a connector, such as C2V or C3, which subsequently responds to the remaining `two digits in the number `to complete the connection to the called line.

In a similar manner, when a call is originated in the tandem oiiice 33 and intended for an outlying oice, for example oiiice 22, the local selector L in oiiice number 33 responds to the first oiiice digit 2 and selects an outoing trunk line through a repeater such as to an incoming selector I in oiiice 22. The

-selector I responds to the second office digitv 2 by raising its wipers opposite the second level, but as is indicated by the numeral 2 placed inside the .circle just above the selector I the wipers of this selector are vreleased and restored to normal whenever they are brought opposite the second level of bank contact responsive to'the dialling of the digit 2. The selector I responds to the next digit, either a 6 or a 7, and extends the connection to a connector, such as the connector C or the connector C', which operates in response to the two remaining digits i/n the number to complete the connection to. the desired line.

At this point it may be mentioned that, whereas the addition of the second oiiice digit to the respective designations of the oiiices other than oiiice 33 is for the .purpose of making the numbers universal so that they will be the same regardlessof from what oiiice they4 arecalled, the addition of the second diglt to the designation of oiiice number 33 is for another purpose; namely, to bring the number of digits inthe numbers of lines in oiiice number 33 up to the five-digits required in the numbers of the lines inthe other oiiice.

Considering now local calls in any o iice, since the drop-back selectors used are arranged so that they drop back repeatedly from their respective critical levels, the banks of the local first selectors are multipled dif.

example, when a local call is made in oiiice number 22the local selector L, which is arranged to drop back from the` second level`y l operates in response to the iirst oflice digitf2f` i and then restores its wipers to-normal. .,'Ihisfy operation is repeated when the secondvoiiicel digit 2 is dialled. When .the third digit in' the number (either a 6 ora 7) isf'dialled. the local selector L raises its wipers to the sixth or to the seventh level andselects anidle connector such as C or C', which connector responds to the next two digits in the number to complete the connection to the desired local f line.

rIhe trunking lay-out having been explained',.a more detailedexplanation ofthe operationof the selector L, Fig. 2, will now be given. Agsumin thathe'subscriber at Y substation A, Fig. 1, esiring to converse with va subscriber whose line terminates 'in oilice numbery 33, removes lhis receiver, the line switch-.LS operates in the usual manner'to v seize an idle local selector, for example, the

' mature 15 opens a p'oint in the circuit of release magnet 7 and preparesa circuit foryerf tical magnet '29, and at amature l14 places ground upon the release trunk conductor 27 so as to maintain the line switch LS operated in the usual manner.

When the calling subscriber manipulates` his calling device in accordance with' the first digit 3 in the desired number, line relay 2 of the local selector L falls back three times.`

Each time it does'so, relay 2 closes at armature 13 a circuit throu h-armature 15 and itswork- .ing contact and t e slow actingseries relay-4 vertical magn t 29, the wipers 231-25 are for vertical mgnet 29'. By the operation of and come to rest-opposite raised step by` bank contacts. The slow the third level acting relay 4 1s energized in series with vertical ymagnet 29 and maintains its armature 16 attracted throughout the vertical move ment.' Oli-normal contacts 8'and11 close as soon as the first vertical step is taken, and off-normal contacts 11 complete a circuit for stepping relayv 5 as follows: from ground by way of the grounded release trunk conductor 27', working contact and armature 16, onormal contacts 11, shaft-spring contacts 1'0, and relay 5to battery. Relay 5 energizes and prepares at armature 18 av4 circuit forrotary magnet 30, and at armature 17 it closes a locking circuit for itselr` from ground throu h the resting contact and armature 21, wor ing contact and armature 17, interrupter contacts 12,' olf-normal contacts `11, and shaft v'spring contacts 10.

At the end of the vertical movement, relay 4`fa1ls back. and completes the rotary magnet circuit and opens the initial circuit of relay 5. Relay 5 remains energized over its locking circuit. Rotary magnet '30 energizes from the grounded release trunk conductor 27 through the normally closed contacts controlled by armature 16 and through armature 18 and its working contact and 4advances the wipers 23-25 into engagement withthe first setof bank contacts in theithird level. vNear the end of its stroke, magnet 30 opens the interrupter contacts 12, whereupon Vrelay 5 falls back and opens a'v further point in its own circuit at armatue 17 and opens the circuit of rotary magnet 30 at armature 18. Ro-

tary magnet 30 now lfalls back and closes its mterrupter contacts 12 again.

Furt er operation depends .upon whether the trunk line terminating in the first set of i 'bank contacts is busy A,o'rl idle. If this trunk line is idle switching relay 6 now energizes,

seizing the trunk. If, however the trunk'is encountered busy, there is a ground potential on Vthe busy test contact by test wiper 24,

A4short-circuiting relay- 6. In this case relay 5 is again energized by-tlie ground potential on test wiper 24 through armature 20 and its resting contact, and contacts 12, 11, and 10, whereupon the circuitofrotary-magnet 30 is again closed at armature 18 and the wipers 23-25 take another step.

This alternate operation of relay 5. and magnet 30 continues as described until an idle trunk line is reached, which trunk line it willbe assumedis the one comprising consov ductors 31-33 and extending to the repeater.-

R, Fig. 1. When this idle trunk l1ne`is' reached, relay 5 does not energize again on account of the 4fact that test wiper 24 does not encounter a grounded contact. Therefore, relay 6, being no longer short circuited,

energizes from ground on the grounded release trunk conductor 27 and through contacts 12, 11, and-10 and relay 5. Relay 5 does not operate at this time owin to the relatively high resistance of relay 6. relay 6 removes the ground potential from armature 13 at armature21; thereby openingthe circuit ofthe slow acting release relay 3; opens a point in the test circuit and prepares theholding circuit at armature 20; and at armatures 19 and 22 disconnects conductors 26-and 28 from the windings of line relay 2 and extends them by way of wipers 23 and 25 iso the conductors 31 and 33 of the seized trunk 'The line and release relays (not shown) of the repeater R now energize over'the seized trunk line andthe latter relay places ground on'- release l trunk conductor 32, thereby grounding, through wiper 24 and armatures 20 and its Working contact the release trunk conductor 27 before the said conductor-"27 is ungroundedxby the falling 'back ofthe slow actlng relay 3. v

pon energizing,

the incoming selector I in the calling oflice 1s cuto from the two-Way inter-otlice trunk l y 1'15 Responsive to the seizure of the repeater R,

` ers reach their normal position.

vtant selectorI raises-ts-wiperssoposite the then reoice number 33: has no utllity im t e present third level of bank contacts an vstores its wipers totheir normal position.

When the next digit in the number is dialled, a connector similar Ato C or C' is' seized. The seized connector respondsto the next two digits in the number to complete the connec- 'tion'to the desired line. I j When the conversation is terminated, the calling subscriber replaces his receiver whereupon the 'release trunk conductor 32 is un- .fgrounded by the repeater R, Fig. .1. When this occurs the switching relay 6 ofthe local selector L, Fig. 2, falls'bac-k and'closes at ar.-

y w'mature 21 a circuit through armature 13 and its resting contact, armature 15 and its resting contact, and off-normal contacts 8 for re-- lease magnet 7. Release magnet 7 thereupon` energizes with the result `that wipers 23-25 are restored to their normal posltion in the usual manner. Off-normal contacts 8 open the circuit of release magnet 7 when the wip- Assuming now thatv the subscriber at substation A removes his receiver; that the line switch LS seizes the'selectorL; and that the calling subscriber desires to converse with a y subscriber in his own exchange, the subscriber at substation A dials the first oilicedigit 2, with the result that the wipers 23-25 of the local selector L are raised oposite the second level of bank contacts. When this occurs, the

.. second levelshaft springs, which are operated only when the wipers are standing opthe shaft-spring contacts9. Release-magnet 7 energizes with the result that the wipers 23-25 are restored to their normal position '.in the usual manner. The circuit of release.

magnet 7- is opened'at contacts 9 res onslve to the 'dropping of the shaft. The re easing operation, however, is not stopped responsive Ato the deenergization of release ma et 7 because of the usual so-called release lmk which maintains the so-called double dog in its normal position.

Asv another result of the dropping of the shaft, the usual circuit of switching relay '6 .is completed at contacts 10 and it remains closed untilthe olf-normal contacts 11 open. Relay 6 does not energize at this time on account of the short time taken forth'e shaft to lcomplete its restoration to normal and open a5 out shown in Fig. 1, certain. modifications vwill be* described. It be naad tha the addition of the second 3 to the des' ation of layout other than to make the tele hone numbers of thelines terminating in o ce number 33 of the same length as the telephone numbers of lines` terminating inthe other oices.

In certain'mstances'it maybe permissible,

or even desirable, to have the telephone numbers shorter in ollice number 33, in whichcase the vsecond 3 is merel omitted and the olice becomes instead o ce number'3.` In

the modified arrangement, the drop-back feature is eliminated in the incoming. selectors I', I2, and Ia asv it is the second oilice di 't'- 3 that these' incomin selectors absorb in t e layout as shown. The local selector Lstill contains the drop-back feature 'and jdrops back on local calls only once, the designation of the connectors C2 and C in this case being '36 and 37, res iectiveldyl. Y

v A still furt er mo 'fcati'on may be had by goin a step further. Trunk lines may be run irectly from oilice number 22 to the1 oflices 44 and 55, in which case the second digit in the designation of oce number 22 may be eliminated. A In thisfurthermodification thethird level of the selectors in oiice between the third and fourth levels and between thefourth and fifth levels are cut.

,Trunks from the fourth level `are taken directly through outgoing repeaters to trunk lines leading to incoming selectors in officenumber 44, and trunks fromthe ifth level are similarly extended tooiice number 55.

In oliice number`44, the tiebetween the second and third levels is now removed and the second level trunks leaddirectly to oliice ics number 22. The same is; true in oiiice 4number 55.

In order to obtain direct trunking throughout, it is only necessary in the ollices 44 and 55 to now remove the ties between the third -and fifth and the third and fourth levels 'respectively, and to join the two oliices by direct trunklines. When this lis accomplished all the oliice designations may consist of one digit each and the oflice designations become merely 2,3, 4, and 5 respectively; none of the incoming selectors are ofthe dropnback type;

and the local selectors in the variousv ollices two levels lof the I `to 'theA local oilice.y

In addition to the and seventh levels" thereto of a diierent numerical order, means for. seizing said switch and for transmitting which now'lead to connectors and in addition to the-levels available foruture growth', there, are in each oiice threelou'tgoing levels and one dead level, the latter corresponding jj-ment will nowbe described whereby the lev- A1li els indicatedfas dead levels in the various oiices may each be used to trunk to an addi*-- tional connector group.

' Considering the above described situation in which direct trunks are'run between each 1920. The drop back selector shown in the lMartin application restores its shaft and wipers toA normal when the critical digitis dialled the first time, but when this digit'is i repeated the wipers are again brought to the same level and areiotated in Search of an idle trunk line in th'e usual manner. The connection isthus extended toan idle connector which may be operated to complete the connection to a desired line. l

This same arrangement as regards the local selectors, may be applied to the tandem trunkin layout as shown in Fig; 1 by merely modi-A i yln the selector shown in the above Martin appllcation so that it releases twice from the critical level and then hunt-s on the critical level when the critical digit is dialled thethird time. .In connection with the incoming selectors, however, it is merely necessary to -make them of the type shownvin theabove Martin application as they are never called upon to vabsorb more than one digit each.

What is claimed is: l

1. In a telephone system, an. automatic.

switch, a terminal of a given order accessible thereto, another terminal accessible thereto of a numerical order two orders removed from. the iirst'named terminal, means for seizing said switch and for transmitting series of impulses thereto, said switch being responsive to a series of impulses 0j one value Y to select the first named terminal and responsive to a series of impulses of another value to select said other terminal, and means in said switch' operative subsequent to the seizure thereof and prior to the-receipt of the last named series of impulses for cancelling two series of impulses.

' 2. In a telephone system, an: automatic switch, aj terminal of a given numerical order accessible thereto, another terminal accessible Obviously, connector. groups cannot be assigned toithose levels I'. eading to outgoing trunk s,.but an arrangeyoiliee for absorbin series of lmpulses thereto, said switch being responsive to a serles of impulses representing a given digit for seizing the first named.

terminal and responsive to impulses representing a different digit for seizin said other terminal, and means in said swit operative responsive to 'the receipt of impulses representin a still diierent digit for cancelling such git priorv to the receiptofthe second named series of impulses. I

3. In a telephone system, a rst oiiice, -a

second piice, and a third oice, switching mechamsm in the first oiice responsive to one digit, for trunln'ng a call from the first oii'ce to the second oiiice, switching mechanism in the second office responsive to a second digit for extendin the connection from the second oiiice to the t ird oiice,and lmeans associated with the switching mechanism in the second the rst oiiice and lntendedA for. a line terminatin in the second office.

4. n a telephone system, a first office, a second oiice, and a third oice, switching mechanism in the irst oflice responsive to one digit for trunkin a call from the flrst oiiice to the second o ce, switching'mechanism in the second oiice responsive to a second digit for extendin theconnection from the second office to the t ird oiiice, means associated with the switching mechanism in the second oice for absorbingone digit on calls fromthe first oflice and intended for aline terminating in the second oiiice, and lmeans associated with the switching mechanisml in .the irst oflice for absorbing two digits on calls 'local to the first oitice. A

5. In a telephone system, a first oiiice, a second `oce, and a thirdvoiiice, the third oce being designated by means of two digits, switching mechanism in the `irst office responsive to the iirst of the two said di -ts to extend a connection to the second o ce, switching mechanism in the second oce responsive to the second of said digits for extending said connection to the third oiiice,

other switching mechanism in the second offlce responsive inl a second call t0 the first digit in said oiiice desi ation for extending a connection to the t ird oiiice, switching mechanism in the third oiiice,`,and means associated with the said switching mechanism in the third office for cancelling the second digit in the oiice designation 1n the latter case.

6. In a telephone system, a first oiice, a second oiiice, and a third oiiice', the third oice being designated by means oftwo digits,` switchingmechanism in theA rst oiiice re sponsive to theirst of the two said digits to extend a connection to the second cnice, switching mechanism in the vsecond oice responsive to the second of said digits for exone digit on callsfrom I l n vtending said connection to the third oflice,

other switching vmechanism in the second oflice vresponsive in a -second call to the first digit inl said oiiice designation for extending a connection to the third ofiice, switching.

mechanism in the third ofiice, and means associated with said switching mechanism .1n the third office for cancelling the second dlglt 1n the oifice designation in the latter case and for cancelling both'digits in the office deslgnatlon in a local call within the third ofiice.

7. In a telephone system, al first office and a second office, the second ofiice being designated by a given digit, switching mechanism in the rst ofiice responsive to such digit to extend a lconnection tothe second ofiice, and

switching mechanism in the second office rebeing responsive to the 'digit designating the second oflicef-to extend a connection to the second oflice through the corresponding terminal, said selector beingresponsive to the digit designating the local ofiice to cancel such digit wherebyit is enabled to respond Vto the following digit to extend the connection through said other terminal. y

9. In a telephone system comprising three offices, subscribers 'lines designated by numbers all having the same number of digits,

means comprising only two directive switches directly controlled lfrom the calling line for lconnecting two lines in the first office, means comprising only three directive switches similarly controlled for'connecting -a line in vthe first oliice with a line in the second ofiice,

and means comprising four directive switches for connecting a line in the first oflice with a line in the third ofiice. g

1Q. In a telephone system comprising three ofiices, subscribers lines designated by num bers all having the same number of digits,

and three switch trains'each comprising a different number of' switches controlled directly from the calling line inv accordance with the propertelephone numbers for connecting a line in the first oiiice with lines in the first, second and third offices, respectively.

11. In a telephone trunking system, three offices, a selector in the first ofiice, a trunk line extending from the bank of said selector to an incoming selector in the second oflice, la trunk line extending from the bank of said incoming selector to an incoming selector in the third ofiice, and trunk lines extending from the bank of each of said select ors to local connectors.

' 12.In atelephone trunkixlg ofiices, a selector in the first o ce, a trunk line extending from the bank of said selector to an incoming selector in the second oflice, a trunk line extending from the'bank of said incoming vselector to an'incoming selector in the third oce, and local switches of the same numerical order directly accessible from thev bank of each of said selectors.

13. In an'automatic selector switch, meansI system, three I for restoring saidswitch-to normal whenever it is operated in accordance withV a given digit in a telephone number, and meansfor maintaining the switch in operated position whenever it is operated in accordance lwith a different digit.

14. In an automatic selector switch, means for repeatedly operating and automatically releasing the switch lresponsive to' successive digits of the same value, and means for maintaining the. switch in operated position the first time it is operated responsive to a digit of a different value. Y

.15. In an automatic telephone system in which the subscribers telephone numbers all contain the same number of digits, a plurality of ofiices, one of which is atandem office. trunking -arrangements and automatic switches directly responsive to the subscriblers calling devices for establishing local office connections, inter-oiliceconnections over di rect trunks, and tandem inter-office connec tions. there being a different number ofl swltches involved in each type'of connection.

and means associated with certain of the said switches for absorbing theunnecessary digits in calls which require less than the maximum number of switches, v

16. In a telephone system of the type in which automatic switches are arranged in a plurality of switching stages corresponding to the successive digits of telephone numbers, a selective switch equipped with a single set of wipers for extending a connection, and

means for operating said switch in each ofthe first three switching stages. 18' In a switching mechanism having directive primary and automatic secondaryv operations, a relay for controlling the secondary operation, an energizing and locking circuit for said relay, and mechanically operated contacts operated 'responsive to the primary movementof the switching mechanism no?. of wipers for extending a connection, and

to open said circuit and to close another circuit'for initiating the release of the switching mechanism.

19. In a switching mechanism having directive primary and automatic secondary operations, a relay energized responsive to the primary operation of the switching mechanism and. locked in operated position responsive to its own operation, said relay con\ trollin the secondary operation of the switching mechanism, a release magnet, and mechanically operated contacts operated responsive only to the primary operation of the switching mechanism for opening the circuit of said relay land for closing the circuit of said release magnet.

20. In a telephone system of the type in. which automatic switches are used. to eect n connections, a calling line and a called line terminating in the same exchange, a calling device for sending series of digit impulses corresponding to the digits of' a called telephone number, and means whereby sending any one of a plurality of series comprising differing 'numbers of digits will operate a train of automatic switches to extend a connection between said calling line and said called line, said train comprising the same 'switches' in each case.

21. In a telephone system of the type in which connections are completed responsive to the calling of a series of digits corresponding to a called telephone number, a calling l line and a called line terminating in the same exchange, automatic switches, and' means whereby a connection may be completed between said calling line and said called line via the same automatic switches responsive to the calling of any one of a plurality of series of digits, each series comprising a different number of digits.

22. In a telephone system of the type in to thecalling of a series of digits corresponding Ato a called telephone number, a first oflice and a second oiice, a called line in each oiiice, means whereby a local connection to a called line in either office may be completed which connections are completed responsive responsive to the calling of any one of three differing seriesof digits, and means whereby an interoliice connection to alcalled line in either oiiice may be completed responsive to the calling of either of two differing series l of digits.

23. In a telephone system, a line which can be called from other lines in the same exchange by a plurality of call numbers each consisting of a different number of directive digits, automatic switches responsive to said digits, and means whereby the same switches may be operated responsive to any one of said pall numbers to extend a connection to said 24. In a telephone system, a line which can be called by a plurality of call numbers 

